Copper particle clusters and powder containing the same suitable as conductive filler of conductive paste

ABSTRACT

Copper particle clusters constituting a powder suitable for making a conductive paste are provided that are individually composed of not fewer than two and not more than 20 unit particles joined through neck portions. A conductive paste made from the powder is excellent in conductivity. A conductive filler for conductive paste is provided that consists essentially of a mixture of copper particle clusters A individually composed of two or more unit particles joined through neck portions and spherical metallic particles B of smaller diameter than the particles A. A conductive paste made from the filler is low in viscosity and excellent in conductivity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to copper particle clusters and powder containingthe same suitable for use as the conductive filler of a conductivepaste.

2. Background Art

In the fabrication of thick-film circuit boards and the like byscreen-printing a conductive paste on an insulating board, thesilver-system paste long used as the main conductive paste has recentlybeen replaced to some extent by copper-system paste. Copper-system pastehas certain advantages over silver-system paste, including higherresistance to migration, excellent soldering tolerance, and low cost. Acopper-system conductive paste having these advantages is obtained bydispersing copper powder of a particle diameter of about 0.1-10 μm in avehicle (resin binder).

Copper paste using copper powder as metallic filler has also drawnparticular attention in connection with the recent practice offabricating multi-layer boards for high-density component mounting byforming stacked boards with through-holes or via holes in the shape offine holes or slits, charging conductive paste into these openings, andsolidifying the conductive paste by heating to form high-densityconductive circuits in the boards. Copper paste can be used to advantageas the conductive paste charged into the small hole-like or slit-likeopenings.

Known processes of producing copper powder include the mechanicalpulverization process, atomization process of spraying molten copper,electrolytic cathode deposition process, vapor deposition process andthe wet reduction process. Although each method has its merits anddemerits, the wet reduction process is the main one used to producecopper powder for conductive paste because it enables relatively readyproduction of fine powder of a particle diameter suitable for use in aconductive paste. Copper powder production processes using the wetreduction method are taught by, for instance, Japanese PatentPublication JPA No. Hei 4-116109 (1992), JPA No. Hei 2-197012 (1990) andJPA No. Shou 62-99406 (1987).

Although copper-system paste rates highly in various aspects ofperformance, its most basic requirement for use as a conductive paste isexcellent conductivity. When a metallic filler of a given purity isdispersed in a resin at a given filling rate, the electrical resistanceof the resulting paste will nevertheless exhibit different valuesdepending the particle size distribution and particle shape. In order toobtain a paste with low electrical resistance, it is obviously importantto ensure close contact among the particles, i.e., to ensure that themetallic particles are dispersed in the resin at a high filling rate soas to increase the contact interface among the particles. Althoughobvious, this is hard to achieve in practice while also maintaining theother qualities required by the conductive paste, such as good viscosityproperty.

Low electrical resistance can generally be obtained when the metallicfiller in the resin includes many particles with irregular surfacesbecause the surface irregularities increase the contact area among theparticles. However, a paste containing a large percentage of particleswith pronounced surface irregularities is high in viscosity andtherefore difficult to charge into the through-holes or via holes.Simultaneous reduction of electrical resistance and viscosity istherefore difficult because an attempt to lower electrical resistance bycontrolling particle shape has the adverse effect of increasing pasteviscosity.

A first object of the present invention is to provide copperparticles/powder that exhibit high conductivity (low electricalresistance) when dispersed in a resin.

A second object of the present invention is to provide copperparticles/powder that exhibit high conductivity (low electricalresistance) and minimize viscosity increase when dispersed in a resin.

SUMMARY OF THE INVENTION

The present invention achieves the first object by providing copperparticle clusters for conductive paste individually composed of two ormore unit particles, preferably 2 to 20 unit particles, joined throughneck portions. The individual copper particle clusters for conductivepaste provided by the present invention are preferably composed 2 to 20unit particles of about 0.5-10 μm diameter joined through neck portionsin arbitrary directions in three-dimensional space. A “neck portion” isdefined as a portion through which two unit particles are joined whichis of a diameter smaller than the diameter of at least one of the unitparticles joined thereby, preferably smaller than the diameters of bothunit particles joined thereby.

A metallic copper powder composed of such copper particle clusters canbe advantageously produced by a process for producing copper powdercomprising a step of precipitating copper hydroxide by reacting anaqueous solution of a copper salt and an alkali to obtain a suspensioncontaining copper hydroxide, an intermediate reduction step effected byadding a reducing agent to the suspension to reduce the copper hydroxideto cuprous oxide, and a final reduction step of reducing the cuprousoxide in the suspension to metallic copper using a reducing agent, inwhich process the copper hydroxide precipitating step is conducted underan atmosphere of an oxygen-containing gas, the copper hydroxideprecipitating step is conducted in an aqueous solution of an Feconcentration of not greater than 50 ppm, and an oxygen-containing gasis blown into the suspension containing cuprous oxide after theintermediate reduction step.

The present invention achieves the second object by providing a copperpowder for conductive paste (conductive filler) composed of copperparticle clusters with neck portions and metallic particles without neckportions. More specifically, the invention provides a filler forconductive paste capable of simultaneously lowering electricalresistance and viscosity consisting essentially of a mixture of copperparticle clusters A individually composed of two or more unit particlesjoined through neck portions and spherical metallic particles B ofsmaller diameter than the particles A. The spherical metallic particlesB are preferably mixed with a copper powder comprising the copperparticle clusters A at a rate such that the weight ratio of B to A (B/A)is in the range of 1/19 to 3/5, and the ratio of the average particlediameter D_(A) of the copper particle clusters A to the average particlediameter D_(B) of the spherical metallic particles B (D_(A)/D_(B)) is inthe range of 5/4 to 8/1. The spherical metallic particles B can becopper particles or copper particles coated with silver. The averageparticle diameter D_(A) of the copper particle clusters A is preferablyin the range of 4-8 μm. The copper powder comprising the copper particleclusters A is still more preferably one that has been subjected tosurface smoothing treatment. The surface smoothing can be achieved bycausing mechanical contact among the copper particle clusters A. Whenthe copper particle clusters A are ones that have been subjected tosurface smoothing treatment, the copper particle clusters may be presentwhose neck diameter is greater than the diameter of at least one of theunit particles at opposite ends of the neck portion. The second objectcan also be achieved in this case because the improved fluidity of thefiller produced by the surface smoothing reduces the paste viscosity.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope (SEM) image of metallic copperpowder obtained in Example 1 according to the invention.

FIG. 2 is an enlarged SEM image of particles at the middle of FIG. 1.

FIG. 3 is an SEM image of metallic copper powder obtained in ComparativeExample 1.

FIG. 4 is an enlarged SEM image of particles at a location slightlyabove the middle of FIG. 3.

FIG. 5 is an SEM image of mixed powder No. 4 obtained in Example 2according to the invention.

FIG. 6 is an SEM image of mixed powder No. 6 obtained in Example 2.

FIG. 7 is an SEM image of mixed powder No. 7 obtained in Example 2.

FIG. 8 is an SEM image of mixed powder No. 10 obtained in Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 are scanning electron microscope (SEM) images of ametallic copper powder in accordance with the present invention obtainedin a working Example set out later. FIG. 2 is an enlarged SEM image ofparticles at the middle of FIG. 1. The copper particle cluster at thecenter of FIG. 2 can be considered to consist of 7 unit particles ofabout 2.4-5 μm diameter joined in random directions in three-dimensionalspace via neck portions. The diameters of the neck portions are smallerthan the diameters of the unit particles located at their opposite ends.As can be seen in FIG. 1, this copper particle cluster composed of 7unit particles is one among various sized copper particle clustersconsisting of 1, 2, 3 . . . n unit particles (1 indicating an individualunit particle not joined to any other and n having a maximum value ofaround 20 in FIG. 1). The unit particles of the multi-particle clustersare not in mere point contact but are interconnected through neckportions constituting a certain amount of connection area. Themechanical strength of the neck portions is sufficient to preventseparation of the unit particles joined thereby when the copper particleclusters experience impacts of a degree occurring during normalhandling, such as when the powder flows during pouring or the like. Aconductive paste prepared from a copper powder containing copperparticle clusters joined via such neck portions was found to exhibitmarkedly lower electrical resistance than one not containing such copperparticle clusters.

FIGS. 3 and 4 are scanning electron microscope (SEM) images of ametallic copper powder obtained in a Comparative Example set out later.FIG. 4 is an enlarged SEM image of particles at a location slightlyabove the middle of FIG. 3. This copper powder can be considered not tocontain copper particle clusters with neck portions but to consist ofapproximately spherical (ball-shaped) particles of 2-10 μm diameter. Thedistribution of the particles is substantially uniform and the averageparticle diameter is about 6.0 μm.

Pastes prepared by dispersing equal amounts of the metallic copperpowders in equal amounts of the same type of resin were measured underidentical conditions for electrical resistance (volume resistivity ofdried coating). As indicated in the examples set out later, the pasteprepared from the metallic copper powder shown in FIGS. 1 and 2(hereinafter called the “former paste” and “former powder) exhibitedelectrical resistance of 3.12×10⁻³ Ω·cm, while that prepared from themetallic copper powder shown in FIGS. 3 and 4 (hereinafter called the“latter paste” and “latter powder”) exhibited electrical resistance of2.76×10⁻² Ω·cm. The former paste thus exhibited electrical resistance ofan order of magnitude smaller than the later and was therefore excellentin conductivity.

The reason for this is not completely clear but is thought to be thatthe certainty of the connectivity among the unit particles of themetallic copper powder including copper particle clusters was greater inproportion to the number of copper particle clusters contained therein,so that better overall filling of the resin was achieved owing toincreased contact interface among the particles and also to the presenceof smaller copper particle clusters and individual unit particlesbetween larger copper particle clusters. However, a metallic copperpowder similar to the former powder except that it comprised copperparticle clusters composed of more than 20 interconnected unit particleswas found to be undesirable because it agglomerated owing to poordispersibility in the resin.

It was learned that a metallic copper powder which, like the formerpowder, has copper particle clusters possessing an appropriate number ofneck portions distributed therein can be produced according to themetallic copper powder production process employing wet reduction, bycontrolling the atmosphere in the initial step for forming copperhydroxide and further controlling impurity content in the step forforming the copper hydroxide. Specifically, it was learned that copperparticle clusters having the aforesaid number of neck portions can beproduced by a modification of the conventional copper powder productionprocess comprising a step of precipitating copper hydroxide by reactingan aqueous solution of a copper salt and an alkali to obtain asuspension containing copper hydroxide, an intermediate reduction stepeffected by adding a reducing agent to the suspension to reduce thecopper hydroxide to cuprous oxide, and a final reduction step ofreducing the cuprous oxide in the suspension to metallic copper using areducing agent, in which modified process the copper hydroxideprecipitating step, conventionally conducted under an inert atmosphereof nitrogen or the like, is instead conducted under an atmosphere of anoxygen-containing gas, typically under an air atmosphere, and the copperhydroxide precipitating step is conducted in an aqueous solution reducedin concentration of co-present impurities such as Mg, Ca, Zn, Na, Al, Feand the like. Moreover, by blowing an oxygen-containing gas, typicallyair, into the suspension of cuprous oxide after reduction to cuprousoxide but before final reduction to metallic copper, the particlediameter and particle size distribution of the obtained copper powdercan be controlled in proportion to the amount of blown gas.

The process will be explained in further detail. First, in the step ofreacting an aqueous solution of a copper salt and alkali to precipitatecopper hydroxide, an aqueous solution of copper sulfate can be used asthe aqueous solution of a copper salt and an aqueous solution of NaOHcan be used as the alkali, as is ordinarily practiced. Alternatively, anaqueous solution of copper chloride, copper carbonate, copper nitrate orthe like can be used as the aqueous solution of a copper salt and any ofvarious other alkalis that have no effect on other aspects of theinvention process can be used as the alkali. The reaction forprecipitating copper hydroxide can be conducted by the method ofseparately preparing an aqueous solution of copper salt of a certainconcentration and an aqueous alkali solution of a certain concentration,mixing the two and then immediately subjecting the mixture to vigorousstirring. Otherwise it can be carried out by the method of continuouslyadding the aqueous alkali solution to the aqueous solution of a coppersalt under stirring. When the copper hydroxide is precipitated under anoxygen-containing gas atmosphere (e.g., air) instead of under theconventional inert gas atmosphere, the intermediate reduction producescuprous oxide of a relatively large diameter, typically of a particlediameter of about 0.5-1.5 μm. In contrast, when copper hydroxide isproduced under the same conditions except that the precipitation isconducted under a nitrogen atmosphere, cuprous oxide of a small particlediameter of around 0.3 μm is obtained. When the particle diameter of thecuprous oxide is so small, a metallic copper powder with neck portionsas described above is difficult to obtain.

It is also difficult to obtain cuprous oxide of such large diameter whenthe concentration of impurities such as Mg, Ca, Zn, Na, Al and Fe in thesuspension is high. Fe present in the suspension acts especiallystrongly to prevent formation of large-diameter cuprous oxide particles.The content of these impurities is therefore best reduced to as low aspossible. Fe concentration is preferably reduced to a concentration ofnot greater than 50 ppm and overall impurity concentration is preferablyreduced to not greater than 70 ppm, more preferably not greater than 50ppm.

As such impurities are ordinarily included in the copper salt used as astarting material, a copper salt of the highest purity feasible ispreferably selected.

Addition of a reducing agent to the suspension of copper hydroxidesuspension in order to reduce the copper hydroxide to cuprous oxide(intermediate reduction) can be conducted by using a glucose as thereducing agent. This intermediate reduction step is preferably carriedout under an inert gas atmosphere and increasing temperature. Aftercompletion of the intermediate reduction treatment, preferably theatmosphere is replaced with an oxygen-containing gas andoxygen-containing gas is bubbled into the suspension.

While this oxidation treatment conducted after the intermediatereduction makes the pH of the suspension 5-9, the diameter of the copperunit particles at the time of final reduction tends to increase withincreasing amount of blown-in oxygen-containing gas. As the amount ofblown-in oxygen-containing gas depends on the flow rate and the blow-inperiod, the diameter of the unit particles can be controlled byadjusting the flow rate and the blow-in period. It was found that unitparticles of more uniform diameter, i.e., of a narrow particle sizedistribution, can be obtained when this oxidation treatment is conductedthan when it is not. It was also found that the treatment results in theformation of unit particles of ball-like shape. The amount of blown-inoxygen-containing gas required to achieve these effects can be obtainedby adjusting the flow rate and the blow-in period so that the amount ofoxygen becomes at least 0.1 mole per mole of copper in the suspension.Although there is no particular upper limit on the amount of blow-in,the effect of oxygen-containing gas blow-in eventually saturates as theamount increases. From the practical viewpoint, and depending on theblow-in method, therefore, an amount of oxygen of not more than 20moles, or in some cases not more than 10 moles, per mole of copper inthe suspension is appropriate. Use of air as the blown-inoxygen-containing gas is most convenient. In the absence of specialcircumstances, blowing room-temperature air into a room-temperaturesuspension suffices. Oxygen-enriched air or pure oxygen is, of course,also usable.

The suspension is next decanted under an inert gas atmosphere to removethe supernatant and harvest the precipitants. The precipitants are thenre-suspended in water and final reduction to metallic copper is carriedout using hydrazine hydrate as the reducing agent. The metallic copperobtained is separated from the liquid and dried, after optional surfacetreatment for imparting oxidation resistance, to afford a metalliccopper powder comprising particles having neck portions.

The present invention thus provides a process for producing copperpowder composed of metallic copper particles having neck portionscomprising a step of precipitating copper hydroxide by reacting anaqueous solution of a copper salt and an alkali to obtain a suspensioncontaining copper hydroxide, an intermediate reduction step effected byadding a reducing agent to the suspension to reduce the copper hydroxideto cuprous oxide, and a final reduction step of reducing the cuprousoxide in the suspension to metallic copper using a reducing agent, inwhich process the copper hydroxide precipitating step is conducted underan atmosphere of an oxygen-containing gas, the copper hydroxideprecipitating step is conducted in a solution of an Fe concentration ofnot greater than 50 ppm, and oxidation treatment is conducted afterintermediate reduction to cuprous oxide by blowing an oxygen-containinggas into the suspension.

This method enables production of a copper powder comprising copperparticle clusters each composed of two or more unit particles joined byneck portions. As this “necked” copper powder makes a paste excellent inconductivity, it can be advantageously used to produce a conductivepaste for use in applications that place utmost priority on conductivityimprovement. As such, it fulfills the first object of the presentinvention.

However, such a copper powder composed solely of copper particleclusters, particularly one that, as illustrated in FIGS. 1 and 2, iscomposed solely of copper particle clusters whose neck portions aresmaller in diameter than the unit particles, exhibits inferiordispersibility in resin and a paste produced from the copper powdertherefore generally has higher viscosity than a paste produced from acopper powder without necks. Such a copper powder is therefore notappropriate for applications in which paste fluidity is of primaryimportance. For instance, it is not appropriate for a paste to becharged into the through-holes or via holes of stacked circuit boardsbecause the paste has poor filling property with respect to such fineholes.

Further research was therefore conducted regarding how to improve thedispersibility in resin of a copper powder composed of copper particleclusters having neck portions while maintaining the conductivityimproving effect thereof to the utmost possible (i.e., how to achievethe second object of the invention). As a result, it was discovered thatthe second object of the invention can be achieved by mixing a copperpowder composed of copper particle clusters having necks (copperparticle clusters/powder A) and spherical metallic particles of smalldiameter, e.g., spherical copper particles or copper particles coatedwith silver (spherical metallic particles/powder B), while appropriatelycontrolling the average particle diameter and mixing ratio of the twotypes of particles.

It was further found that still better results can be obtained by mixingthe spherical metallic powder B with a necked copper powder A composedof smoothed clusters whose angular surface irregularities have beenremoved by surface smoothing treatment beforehand. The surface smoothingtreatment of the necked copper powder is preferably conducted by themethod of causing mechanical contact among the copper particle clustersA. As set out in the working examples explained later, a cylindricalhigh-speed agitator, for example, can be advantageously used for thispurpose. When the copper particle clusters A are ones that have beensubjected to surface smoothing treatment, the copper particle clustersmay be present whose neck diameter is greater than the diameter of atleast one of the unit particles at opposite ends of the neck portion.

In implementing this aspect of the invention, a paste that exploits theexcellent conductivity of necked copper powder and also exhibitsmarkedly low viscosity can be obtained by producing copper powderconsisting of copper particle clusters A individually composed of two ormore unit particles joined through neck portions, mixing the copperpowder consisting of copper particle clusters A and a spherical metallicpowder consisting of spherical metallic particles B of a smallerdiameter than the copper particle clusters A at a rate such that theweight ratio of B to A (B/A) is in the range of 1/19 to 3/5 and theparticles B can enter spaces between the particles A. Particularlypreferable is to establish a relationship wherein the average particlediameter D_(A) of the particles A is in the range of 4-8 μm, preferably5-7 μm, and the average particle diameter D_(B) of the sphericalmetallic particles B is such that D_(A)/D_(B) is in the range of 5/4 to8/1, preferably 5/3 to 7/1. When the ratio falls below this range, thepaste viscosity lowering effect obtained by mixing in the sphericalmetallic particles B is low and the electrical resistance is high. Whenthe ratio is above this range, the paste dispersion property is poorbecause the particles B are of such small diameter as to readily giverise to agglomeration.

When the weight ratio B/A is smaller than 1/19, the spherical metallicparticles B do not produce a sufficient paste viscosity lowering effect,and when it is greater than 3/5, the electrical resistance rises to alevel that diminishes the technical advantage of using the copperparticle clusters A. Typically, the average particle diameter D_(A) ofthe particles A is 4-8 μm and the average particle diameter D_(B) of thespherical metallic particles B is not less than D_(A)×1/7 and notgreater than D_(A)×3/5, and the weight ratio B/A is preferably in therange of 1/19 to 3/5.

The spherical metallic particles B are preferably spherical copperparticles or spherical silver-coated copper particles. A particularlymarked reduction of electrical resistance can be realized by usingspherical silver-coated copper particles. The spherical silver-coatedcopper particles can be obtained by coating the surfaces of sphericalcopper particles with silver. The silver coating can be effected usingsilver complex salt solution with EDTA (ethylenediamine-tetraaceticacid) or by the method of adding silver nitrate to the suspensionincluding suspended spherical copper particles obtained in the finalstep of the wet reduction process. Spherical copper particles withoutneck portions can be produced by conducting the initial step for formingcopper hydroxide under an inert atmosphere instead of under theoxidizing atmosphere used when producing necked copper powder. Theaverage diameter of the spherical copper particles can be controlled bycontrolling the amount of blown-in air in the bubbling step. Thisenables production of spherical copper powders of various averageparticle diameters and uniform particle size.

“Spherical metallic particles B” as termed with respect to the presentinvention is, as is usual in this technical field, defined to includenot only perfectly spherical bodies but also bodies of overall spherical(ball-like) shape as distinguished from particles of other shapes suchas plate or needle like, and it should be understood that the termencompasses ball-like bodies whose surfaces exhibit irregularitiesand/or angular protrusions to some degree insofar as they are sphericalas a whole.

WORKING EXAMPLES Example 1

The following aqueous solution of copper sulfate A and aqueous solutionof alkali B were prepared.

Aqueous Solution of Copper Sulfate A:

-   -   (CuSO₄.5H₂O: 0.6925 Kg)+(Pure water: 2.20 Kg)        Aqueous Solution of Alkali B:    -   (Aqueous solution of NaOH of 48.1% concentration: 0.545        Kg)+(Pure water: 4.15 Kg)

The total amount of aqueous solution of copper sulfate A held at 29° C.was added to the aqueous solution of alkali B held at 27° C. in an airatmosphere and stirred. The Fe concentration of the solution was nogreater than 50 ppm and other impurities were present only in traceamounts. The temperature of the A+B solution was increased to 32° C. byheat of the reaction. A suspension containing precipitated copperhydroxide was obtained. The pH of the solution was 13.2. The mixingratio of the A solution and B solution was such to make NaOH present ata chemical equivalent ratio of 1.20 relative to copper contained in thesolution.

A glucose solution prepared by dissolving 0.9935 Kg of a glucose in 1.41Kg of pure water was added to the total amount of copper hydroxidesuspension obtained. The solution rose to a temperature of 70° C. over a38-min period following the addition and was maintained at thistemperature for 15 min thereafter. The pH of the suspension at this timewas 7.8. This step was conducted under a nitrogen atmosphere.

The suspension was then oxidized by bubbling air into it at a flow rateof 0.7 liter/min over a period of 420 min. The pH of the suspensionbecame 5.76 as a result. The diameter of the cuprous oxide particles wasabout 0.7 μm.

The suspension was left standing in a nitrogen atmosphere for two days.The supernatant (pH 5.99) was then removed to harvest substantially thetotal amount of the precipitated cuprous oxide. A suspension wasprepared by adding 0.55 Kg of pure water to the precipitate. 0.074 Kg ofhydrazine hydrate was added to the total amount of the suspension inseveral lots. The temperature of the suspension rose from 50° C. to afinal temperature of 80° C. owing to heat generated up to completion ofthe reaction. Upon completion of the reaction, the suspension wassubjected to solid-liquid separation and a copper powder was obtained bydrying the harvested solid content at 120° C. under a nitrogenatmosphere.

An SEM image of the copper powder obtained by this method is shown inFIGS. 1 and 2. As explained earlier, the copper powder contained copperparticle clusters individually composed of unit particles of 2.5-5.0 μmdiameter joined through neck portions. Based on a count of the copperparticle clusters in the SEM, about 40% of all particles were accountedfor by the copper particle clusters.

A paste was prepared in accordance with the conventional method ofpreparing a copper-system conductive paste, by kneading together 30 g ofthe copper powder and 7.5 g of phenolic resin as a thermosetting resin.A 30 μm-thick coat of the blended material was applied to a glass plateand dried. The volume resistivity of the coat was 3.12×10⁻³ Ω·cm.

Comparative Example 1

The following aqueous solution of copper sulfate A and aqueous solutionof alkali B′ were prepared.

Aqueous Solution of Copper Sulfate A:

-   -   (CuSO₄.5H₂O: 0.6925 Kg)+(Pure water: 2.20 Kg)        Aqueous Solution of Alkali B′:    -   (Aqueous solution of NaOH of 49.0% concentration: 0.541        Kg)+(Pure water: 4.15 Kg)

The total amount of aqueous solution of copper sulfate A held at 29° C.was added to the aqueous solution of alkali B′ held at 27° C. in anitrogen gas atmosphere and stirred. The Fe concentration of thesolution was no greater than 50 ppm and other impurities were presentonly in trace amounts. The temperature of the A+B′ solution wasincreased to 32.9° C. by heat of the reaction. A suspension containingprecipitated copper hydroxide was obtained. The pH of the solution was12.9. The mixing ratio of the A solution and B′ solution was such tomake NaOH present at a chemical equivalent ratio of 1.19 relative tocopper contained in the solution.

A glucose solution prepared by dissolving 0.9935 Kg of a glucose in 1.41Kg of pure water was added to the total amount of copper hydroxidesuspension obtained. The solution rose to a temperature of 70° C. over a38-min period following the addition and was maintained at thistemperature for 15 min-thereafter. The pH of the suspension at this timewas 7.8. This step was conducted under a nitrogen atmosphere.

The suspension was then oxidized by bubbling air into it at a flow rateof 0.7 liter/min over a period of 420 min. The pH of the suspensionbecame 5.80 as a result. The diameter of the cuprous oxide particles wasabout 0.3 μm.

The suspension was left standing in a nitrogen atmosphere for two days.The supernatant (pH 6.02) was then removed to harvest substantially thetotal amount of the precipitated cuprous oxide. A suspension wasprepared by adding 0.55 Kg of pure water to the precipitate. 0.074 Kg ofhydrazine hydrate was added to the total amount of the suspension inseveral lots. The temperature of the suspension rose from 50° C. to afinal temperature of 80° C. owing to heat generated up to completion ofthe reaction. Upon completion of the reaction, the suspension wassubjected to solid-liquid separation and a copper powder was obtained bydrying the harvested solid content at 120° C. tinder a nitrogenatmosphere.

An SEM image of the copper powder obtained by this method is shown inFIGS. 3 and 4. As explained earlier, the copper powder was composed ofball-like particles of an average diameter of about 6.0 μm. No neckportions were present.

A paste was prepared in exactly the same way as in Example 1, bykneading together 30 g of the copper powder and 7.5 g of phenolic resinas a thermosetting resin. A 30 μm-thick coat of the blended material wasapplied to a glass plate and dried. The volume resistivity of the coatwas 2.76×10⁻² Ω·cm.

Example 2

A necked copper powder comprising copper particle clusters obtained inthe manner of Example 1 was charged into a cylindrical high-speedagitator and subjected to forced-flow treatment for smoothing theparticle surfaces under a nitrogen atmosphere. The cylindricalhigh-speed agitator used for the forced-flow treatment was a mixerhaving two rotary blades at the bottom of a vertically disposedcylindrical vessel. Centrifugal force imparted by the rotating bladescaused the powder to flow upward and its particles to repeatedly collidewith one another during the flow period, thereby smoothing offprotrusions on the particles surfaces. The treatment leveled the angularirregularities present on the unit particle surfaces to afford particlesof an overall rounded shape but caused almost no breakup of the copperparticle clusters by separation of unit particles at the neck portions.As the neck portions remained, the copper particle clusters could bedistinguished from spherical particles.

The average diameter of the copper particle clusters after the smoothingtreatment was 6 μm. The helos particle size distribution of the copperpowder was measured using a helos particle size distribution measuringdevice (Helos H0780; Sympatic Co., Ltd.). Particles of a diameter of 4-7μm were found to account for 70 vol % of all particles. As mostparticles thus fell in the 4-7 μm range centered on 6 μm, the particlesize distribution was very narrow.

Metallic powders No. 1-16 listed in Table 1 were prepared by thoroughlymixing the copper powder composed of the smoothed copper particleclusters with different metallic powders composed of spherical copperparticles or spherical silver-coated copper particles, at the mixingratios indicated in the same table. SEM images of the No. 4, No. 6, No.7 and No. 10 mixed powders, considered representative of the sixteenmixed powders prepared, are shown in FIGS. 5, 6, 7 and 8, respectively.

The spherical powders composed of the spherical copper particles ofdifferent average particle diameter shown in Table 1 were prepared toconsist of ball-like copper particles (i.e., spherical copper particleswithout neck portions) of uniform size by a method similar to that ofExample 1, except that the addition of the aqueous solution of coppersulfate A to the aqueous alkali solution B was conducted under anitrogen atmosphere instead of an air atmosphere and the conditions ofthe air bubbling step were modified. The spherical silver-coated copperparticles were produced by using a solution of silver complex salt withEDTA to deposit a thin film of silver on the surface of spherical copperparticles.

The spherical powders composed of spherical copper particles in column(a) of Table 1 were ones subjected to the surface smoothing treatmentexplained earlier. The spherical powders composed of the sphericalsilver-coated copper particles in column (b) of Table 1 were onesobtained by silver coating copper powders composed of smoothed sphericalcopper particles.

The viscosities (F value=92 or 93) and electrical resistances of pastesin which the No. 1-16 mixed metallic powders were dispersed weremeasured in the following manner. The results of the measurements areincluded in Table 1.

Filler value (F value=92): 92 wt % of the mixed powder was kneaded into8 wt % of epoxy resin in a vibration type mixer and the viscosity of theobtained paste was measured. The epoxy resin used (glycidyl-esterfieddimer acid) had an epoxy equivalent of 446 g/eq and a 25° C. viscosityof 730 mPa·s. The kneading conditions were the same for all copperpowders. Paste viscosity was measured using a B type viscometer at 10rpm and 25° C.

Filler value (F value=93): The viscosity of a paste similarly preparedfrom 93 wt % of the mixed powder and 7 wt % epoxy resin was measured inthe same manner.

Powder resistance: The copper powder was charged into a 2-mm diameterround cylinder made of insulating material and the copper powder wasapplied with a load progressively increased from 0 and 150 kgf. Theresistance to passage of electric current at a load of 25 kgf wasdefined as the electrical resistance value of the copper powder. TABLE 1Copper particle Spherical powder (B) Electrical clusters (A) (a)Spherical (b) Spherical resistance Smoothed copper copper silver-coatedPaste viscosity Powder particle clusters particles copper particles F92value F93 value resistance No. Diameter Wt % Diameter Wt % Diameter Wt %Pa · s Pa · s mΩ Photo Remark 1 6 100 — — — — 127 1041 126 Control 2 690 3 10 — — 65 213 34 Invention 3 6 80 3 20 — — 58 159 113 Examples 4 670 3 30 — — 55 138 220 5 6 95 1.5 5 — — 56 116 31 6 6 90 1.5 10 — — 4679 37 7 6 80 1.5 20 — — 38 57 27 8 6 95 — — 1.5  5 56 116 24 9 6 90 — —1.5 10 48 82 18 10 6 80 — — 1.5 20 35 54 20 11 — — 6 100 — — 74 131 170Comparative 12 — — 6 80 — — 58 115 160 Examples — — 1.5 20 13 6 80 4.520 — — 65 232 199 14 6 80 5 20 — — 116 850 220 15 6 80 10 20 — — 227 25016 6 80 20 20 — — 173 300

The results shown in Table 1 demonstrate that:

(1) No. 1 composed solely of smoothed copper particle clusters Aexhibited lower electrical resistance and higher paste viscosity thanNo. 11 composed solely of spherical particles B (type (a) copperparticles) of the same diameter.

(2) As can be seen from Nos. 2-10, paste viscosity decreased whenspherical particles B (either type (a) or type (b)) of a smallerdiameter were mixed with the copper particle clusters A of largerdiameter.

(3) As can be seen from No. 2 and Nos. 5-10, electrical resistancedecreased when spherical particles B (either type (a) or type (b)) of asmaller diameter were mixed with the copper particle clusters A of alarger diameter.

(4) As can be seen from Nos. 14-16, paste viscosity did not decrease butrather increased and electrical resistance also increased when sphericalparticles B (type (a) copper particles) of an average diameter of 5 μmor greater were mixed with the copper particle clusters A of an averagediameter of 6 μm.

(5) When spherical particles B (type (a) copper particles) of an averagediameter of 3 μm were mixed with the copper particle clusters A of anaverage diameter of 6 μm, paste viscosity decreased with increasingcontent of spherical copper particles B up to 30 wt % content,whereafter the viscosity decreasing effect saturated and electricalresistance increased.

(6) Paste viscosity decreasing effect and powder resistance decreasingeffect were greater when spherical particles B (type (a) copperparticles) of an average diameter of 1.5 μm were mixed with the copperparticle clusters A of an average diameter of 6 μm than when sphericalparticles B (type (a)) of an average diameter of 3 μm were mixedtherewith. In the case of No. 7, for instance, both paste viscositydecreasing effect and electrical resistance decreasing effect were verypronounced.

(7) When spherical particles B of type (b) (spherical silver-coatedcopper particles) were used, decreased paste viscosity was obtained and,in addition, a greater decrease in electrical resistance was realizedthan when using spherical particles B of type (a) (copper particles).

As explained in the foregoing, the present invention provides a copperpowder for conductive paste capable of forming a coating excellent inelectrical conductivity. The present invention also provides aconductive filler for conductive paste that enables simultaneousreduction of both electrical resistance and paste viscosity. Aconductive paste prepared using this conductive filler has low viscosityand exhibits excellent conductivity in the hardened state. When applied,for example, as a conductive paste for charging into the through-holesand via holes of stacked (multi-layer) circuit boards, it thereforeexhibits outstanding properties not obtainable with conventionalconductive pastes. The present invention can therefore be expected tocontribute greatly to improvement of the performance of multi-layercircuit boards for high-density component mounting.

1-4. (canceled)
 5. A process for producing a copper powder includingcopper particle clusters comprising a step of precipitating copperhydroxide by reacting an aqueous solution of a copper salt and an alkalito obtain a suspension containing copper hydroxide, an intermediatereduction stip effected by adding a reducing agent to the suspension toreduce the copper hydroxide to cuprous oxide, and a final reduction stepof reducing the cuprous oxide in the suspension to metallic copper usinga reducing agent, in which process the copper hydroxide precipitatingstep is conducted under an atmosphere of an oxygen-containing gas.
 6. Aprocess according to claim 5, wherein the copper hydroxide precipitatingstep is conducted in an aqueous solution of an Fe concentration of notgreater than 50 ppm.
 7. A process according to claim 5, furthercomprising a step of blowing an oxygen-containing gas into thesuspension containing cuprous oxide after the intermediate reductionstep. 8-15. (canceled)